XPath does not pass brower compatibility checks in InfoPath 2007

Why do the targetField attributes below fail the browser compatibility checker in InfoPath 2007. Is there another way to write the statement so that it will work on Browser Enabled forms?

<xsf:ruleSets>
     <xsf:ruleSet name="ruleSet_1">
          <xsf:rule caption="Rule 1" isEnabled="yes">
               <xsf:assignmentAction targetField="开发者_高级运维my:group1/my:group2[1]/my:field3" expression="my:field1"></xsf:assignmentAction>
          </xsf:rule>
          <xsf:rule caption="Rule 2" isEnabled="yes">
               <xsf:assignmentAction targetField="my:group1/my:group2[2]/my:field3" expression="my:field2"></xsf:assignmentAction>
          </xsf:rule>
     </xsf:ruleSet>
</xsf:ruleSets>

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Is the "my:" prefix defined somewhere in surrounding XML you have not shown us? If not, then that might be a reason. The XML you've shown us has no definition for "my:".